The present invention relates to a process for catalytic cracking of hydrocarbon oils in the absence of hydrogen, and specifically relates to a process for catalytic cracking of petroleum hydrocarbon stocks in the absence of hydrogen to increase simultaneously the yields of diesel oil and liquefied gas.
Liquefied gas is one of the important petrochemical products, of which light olefins are important chemical raw materials of high commercial value. Diesel oil has high heat efficiency and the exhaust tail gas from vehicles running on diesel oil contains less harmful constituents, so it meets the more and more rigorous requirements for envirornental protection all over the world. Thus, following the increase in the number of vehicles running on diesel oil, the market demand for diesel oils is also growing.
Diesel oil comes mainly from fraction oils produced by the primary and secondary processing of crude oil. In the primary processing, i.e. the atmospheric and vacuum distillation, the yield of diesel fractions from crude oil is fixed, so no potential can be tapped. In the secondary processing, catalytic cracking is usually adopted for producing diesel oil. Featuring large-volume treatment and flexible operation conditions, this process of catalytic cracking is an important means for improving the yields of liquefied gas and diesel oil.
CN 1031834A discloses a catalytic cracking process for producing more light olefins. Although this process can produce large quantities of liquefied gas, but its yield of diesel oil is relatively low, generally less than 10 wt %, and moreover it requires a special catalyst and processing unit.
CN 1085885A discloses a method for obtaining higher yields of liquefied gas and gasoline under the following reaction conditions: a reaction temperature of 480xc2x0-580xc2x0 C., a pressure of 130-350 KPa, a WHSV of 1-150 hxe2x88x921, a catalyst/oil ratio of 4-15, and a steam/hydrocarbon stock weight ratio of 0.05-0.12:1. The yield of liquefied gas in the reaction products is 30-40 wt %, but that of diesel oil is comparatively low.
CN 1160746A discloses a catalytic cracking process for raising the octane number of low-grade gasoline fractions, wherein a low-grade gasoline is introduced into a riser reactor through its lower part and the reaction is carried out under the conditions of a reaction temperature of 600xc2x0-730xc2x0 C., a WHSV of 1-180 hxe2x88x921, and catalyst/oil ratio of 6-180, then a high octane gasoline, is mainly obtained. The feedstocks employed in this process are low-grade gasolines, such as straight-run gasoline, coker gasoline and so on, and the yields of liquefied gas and diesel oil in the reaction products are 24-39 wt % and 0.5-2.3 wt % respectively.
U.S. Pat. No. 3,784,463 discloses a process carried out in a reaction system comprising at least two riser reactors, wherein a low-grade gasoline is introduced into one of the riser reactors and catalytic cracking reaction occurs. By this process, the gasoline octane number and yield of liquefied gas are improved. However, this process cannot give higher yield of diesel oil, and it requires that the reaction unit should be revamped by adding at least another riser.
U.S. Pat. No. 5,846,403 discloses a process of recracking of catalytic naphtha to obtain a maximum yield of light olefins. The process is carried out in a riser reactor comprising two reaction zones, namely an upstream reaction zone in the lower part of the reactor and a downstream reaction zone in the upper part. In the upstream reaction zone, the feedstock is a light catalytic naphtha (having a boiling point below 140xc2x0 C.), and the reaction conditions are: an oil-catalyst contact temperature of 620xc2x0-775xc2x0 C., an oil and gas residence time of less than 1.5 sec., a catalyst/oil ratio of 75-150, and the proportion of steam accounting for 2-50 wt % the weight of naphtha, while in the downstream reaction zone, the feedstock is a conventional catalytic cracking stock (having a boiling point of 220xc2x0-575xc2x0 C.), and the reaction conditions are: a temperature of 600xc2x0-750xc2x0 C. and an oil and gas residence time of less than 20 sec. Compared with conventional catalytic cracking, the yields of liquefied gas and light cycle oil (i.e. diesel oil) of this process increase by 0.97-1.21 percentage points and 0.13-0.31 percentage points higher.
CN 1034949A discloses a process for converting petroleum hydrocarbons in which the stocks, ethane, gasoline, catalytic cracking stock and cycle oil, are successively upwardly introduced into a riser reactor through its lowermost part. This process is mainly aimed at producing light olefins, but the total yield of gasoline, diesel oil and liquefied gas decreases.
EP0369536A1 disclosed a process for catalytic cracking hydrocarbon feedstock, in which a hydrocarbon feedstock is charged into the lower part of the riser reactor wherein said hydrocarbon feedstock in admixed with freshly regenerated cracking catalyst, and a recycle portion of a light liquid hydrocarbon stream in charged into the riser zone at a level above the hydrocarbon feedstock charging level. The process operates in such a manner to produce maximum quantities of fuel oil, or alternatively to produce maximum quantities of olefins in different conditions, but can""t increase the yields of diesel oil and of olefins simultaneously.
U.S. Pat. No. 4,422,925 discloses a process for fluidized catalytic cracking hydrocarbon feedstock for producing gaseous olefins, which comprises charging gaseous C2 to C1 rich stock into the lowermost portion of the riser reaction zone to contact with hot freshly regenerated catalyst and charging heavy hydrocarbon stock to an upper section of the riser, and introducing naphtha or gas oil into a section, between said lower and upper sections of said riser. This process can produce high yield of light olefins but the increment of yield of diesel oil is very small.
U.S. Pat. No. 3,894,932 disclosed a method for converting hydrocarbons which comprises passing C3-C4 gaseous hydrocarbon fraction through a lower portion of a riser, introducing gas oil at one or more spaced apart downstream intervals, and introducing C2-C4 hydrocarbon or isobutylene or gas oil through the upper portion of the riser. This method is aimed at producing aromatics and isobutane but can""t increase the yields of diesel oil and liquefied gas simultaneously.
Another method of increasing the yield of liquefied gas is by adding a catalyst promoter to the catalytic cracking catalyst. For example, U.S. Pat. No. 4,309,280 discloses a method of adding a HZSM-5 zeolite in an amount of 0.01-1% by weight of the catalyst directly into the catalytic cracking unit.
U.S. Pat. No. 3,758,403 discloses a catalyst comprising ZSM-5 zeolite and large-pore zeolite (e.g. the Y-type and X-type) (in a ratio of 1:10-3:1)as active components, thereby raising the yield of liquefied gas and the gasoline octane number by a big margin, while the yields of propane and butane are increased by about 10 wt %. Furthermore, CN 1004878B, U.S. Pat. No. 4,980,053 and CN1043520A have disclosed catalysts comprising mixtures of ZSM-5 zeolite and Y-type zeolite as active components, resulting in that remarkable increases in the yield of liquefied gas are achieved. However, this kind of methods is used to mainly increase the yield of liquefied gas by means of modifying the catalysts, while the increase in the yield of diesel oil is less.
The above-mentioned patented processes can only increase the yield of liquefied gas, but cannot increase the yield of diesel simultaneously, or if any, the yield of diesel oil is insignificant. Moreover, some of the above-mentioned patented processes require special catalysts or reaction units, or the existing units should be largely refitted to meet their specific requirements.
The object of the present invention is to provide a catalytic cracking process for increasing the yields of diesel oil and liquefied gas simultaneously on the basis of the prior art.
The present invention relates to a process for catalytic cracking hydrocarbon stocks to increase simultaneously the yields of diesel oil and liquefied gas, carrying out in a riser or fluidized-bed reactor, which comprise:
(a) Gasoline stock, an optional pre-lifting medium, and a catalytic cracking catalyst are charged into the reactor through the bottom of the reactor and they contact in the lower zone of the reactor to produce an oil-gas mixture with a lot of liquefied gases;
(b) The resultant oil-gas mixture and the reacted catalyst from step (a) flow upwardly and contact, in the zone upper than the lower zone of the reactor, conventional catalytic feed introduced from at least two sites having different heights higher than the lower part of on the reactor, to produce an oil-gas mixture with a lot of diesel oils;
(c) The resultant oil-gas mixture from step (b) enters a fractionation system where it is separated into the desired liquefied gas, gasoline and diesel oil products, heavy cycle oil and slurry, wherein the heavy cycle oil and slurry are optionally circulated back to the reactor;
(d) The spent catalyst may pass through steam stripping and enters a regenerator and undergoes coke-burning and then is circulated back for reuse.